The present invention relates to polybenzazole fibers with a fine surface structure or with a defect-free fiber structure, which are suitable as industrial materials, to a shock-resistant member, heat-resistant felt and the like that utilize the fibers thereof.
Polybenzazole fibers have a strength and an elastic modulus that are twice or more those of polyparaphenylene terephthalamide fibers, typical super fibers commercially available at present, and are expected to be next-generation super fibers.
To produce fibers from a polyphosphoric acid solution of a polybenzazole polymer has been well known. For example, spinning conditions are disclosed in U.S. Pat. Nos. 5,296,185 and 5,385,702, washing and drying methods are disclosed in International Publication WO94/04726, and further a heat treatment method is disclosed in U.S. Pat. No. 5,296,185.
However, polybenzazole fibers made by the aforementioned conventional methods generally have an equilibrium moisture content of 0.6% or more, even though they are subjected to heat treatment at 350xc2x0 C. or higher as disclosed in U.S. Pat. No. 5,296,185. This is an obstacle upon applications to the fields of avoiding extreme moisture absorption of fibers, such as an application to a high-performance high-density electronic circuit board for silicon chip mounting.
Nevertheless, polybenzazole fibers are manufactured by the removal of the solvent from a polymer solution, and thus the generation of voids is unavoidable, and the presence of these voids is a factor of increasing water absorbency. On the other hand, although many polybenzazole fibers with a void size of 25 xc3x85 or less in the fibers are proposed (for example, JP 6-240653 A, JP 6-245675 A, JP 6-234555 A, etc.), the production of these fibers is not easily accomplished when taking industrial production in terms of cost, etc. into consideration. Furthermore, the voids are very small, and water once penetrated into the sites is difficult to be removed, which becomes a hindrance for the reduction of moisture absorbency.
Accordingly, the production of polybenzazole fibers of extremely low water absorbency is not completed yet so far.
Thus, the present inventors have studied intensively in order to develop a technology for easily manufacturing polybenzazole fibers having extremely low water absorbency as an organic fiber material or having a high thermal conductivity property.
As a means for realizing the ultimate physical properties of fibers, stiff polymers such as a so-called ladder polymer have been considered. However, such stiff polymers have no flexibility. Then, in order to provide flexibility and processability as organic fibers, an important condition is that a fiber is a linear polymer.
S. G. Wierschke et al. have shown that a linear polymer with a highest theoretical elastic modulus is cis-form polyparaphenylene benzobisoxazole, in Material Research Society Symposium Proceedings Vol. 134, p. 313 (1989). This result was also confirmed by Tashiro et al. (Macromolecules vol. 24, 706 (1991)). Among polybenzazoles, cis-form polyparaphenylene benzobisoxazole has a crystalline modulus of 475 GPa (P. Galen et al., Material Research Society Symposium Proceedings Vol. 134, p. 329 (1989)) and was thought to have an ultimate primary structure. Therefore, it is theoretically concluded that polyparaphenylene benzobisoxazoles should be selected as polymers for material in order to obtain the ultimate elastic modulus.
Fiber production from the polymer is performed by the methods as disclosed in U.S. Pat. Nos. 5,296,185 and 5,385,702, heat treatment thereof is carried out by the method as proposed in U.S. Pat. No. 5,296,185. Yarn obtained by such methods has an equilibrium moisture content of 0.6% or more. In addition, yarn obtained by such methods has a sound wave propagation velocity of at most about 1.3xc3x9710 6 cm/sec. Thus, the present inventors have recognized the necessity for studies on the improvement of these methods, and studied and found out that the desired physical properties are readily industrially attainable by the method as described hereinafter, even if a void size is 25.5 xc3x85 or more diameter in fibers.